Aziridine iron-catalyzed

Aziridines are versatile intermediates in organic synthesis and commonly found in bioactive molecules. The transition metal-catalyzed nitrene transfer to alkenes is an attractive method for the synthesis of aziridines [7]. In 1984, Mansuy and coworkers reported the first example of an iron-catalyzed alkene aziridination in which iron porphyrin [Fe(TTP)Cl] was used as catalyst and PhINTs was used as nitrene source [30]. Subsequently, the same authors demonstrated that [Fe(TDCPP) (CIO4)] is a more efficient and selective catalyst than [Fe(TTP)Cl] (Scheme 20). 

The first reports on iron-catalyzed aziridinations date back to 1984, when Mansuy et al. reported that iron and manganese porphyrin catalysts were able to transfer a nitrene moiety on to alkenes [90]. They used iminoiodinanes PhIN=R (R = tosyl) as the nitrene source. However, yields remained low (up to 55% for styrene aziridination). It was suggested that the active intermediate formed during the reaction was an Fev=NTs complex and that this complex would transfer the NTs moiety to the alkene [91-93]. However, the catalytic performance was hampered by the rapid iron-catalyzed decomposition of PhI=NTs into iodobenzene and sulfonamide. Other reports on aziridination reactions with iron porphyrins or corroles and nitrene sources such as bromamine-T or chloramine-T have been published [94], An asymmetric variant was presented by Marchon and coworkers [95]. Biomimetic systems such as those mentioned above will be dealt with elsewhere. 

The reaction ofimines such as 33 with ethyl diazoacetates yields complex product mixtures consisting ofaziridines and P-enamino esters. When phenyldiazomethane is used as the nucleophilic component in this iron-catalyzed reaction, aziridines such as 34 are obtained in high yield and as single diastereoisomers (Scheme 8.11) [42]. The catalyst is the same Fe(II)-complex that was applied for the preparation of a-formyl ester 23 (cf. Scheme 8.6). 

The iron-catalyzed process can also be performed with iron-PYBOX systems, as reported by Redlich and Hossain [29]. The PYBOX ligands are powerful tools in organic synthesis. However, for the iron-catalyzed synthesis of aziridines (Scheme 9.14), the results are not as convincing as for similar copper-PYBOX systems. Not only are the yields moderate (up to 54%), but also the enantiomeric excesses (up to 49%) are not in a synthetically useful range, which precludes their use in elaborate applications. 

Scheme 9.14 Asymmetric iron-catalyzed aziridine synthesis.

III.B.2), complexes with manganese, chromium, as well as second- and third-row transition metal ions (e.g., ruthenium) oxidation reactions with dioxygen alone or with other peroxides (e.g., ferf-butyl-peroxide) the stabilization and spectroscopic characterization of mononuclear superoxo, peroxo, and oxo complexes other catalytic processes (e.g., the iron-catalyzed aziridination), enantioselective reactions with chiral bispidine ligands and the iron oxidation chemistry continues to produce novel and exciting results. 

The mild iron-based Lewis acid, [(q -CsH5)Fe(CO)2(THF)]BF4 reported by Hossain, catalyzed the aziridination of styrene derivatives with PhlNTs with product... 

A more practical, atom-economic and environmentally benign aziridination protocol is the use of chloramine-T or bromamine-T as nitrene source, which leads to NaCl or NaBr as the sole reaction by-product. In 2001, Gross reported an iron corrole catalyzed aziridination of styrenes with chloramine-T [83]. With iron corrole as catalyst, the aziridination can be performed rmder air atmosphere conditions, affording aziridines in moderate product yields (48-60%). In 2004, Zhang described an aziridination with bromamine-T as nitrene source and [Fe(TTP)Cl] as catalyst [84]. This catalytic system is effective for a variety of alkenes, including aromatic, aliphatic, cyclic, and acyclic alkenes, as well as cx,p-unsaturated esters (Scheme 28). Moderate to low stereoselectivities for 1,2-disubstituted alkenes were observed indicating the involvement of radical intermediate. 

Attempts to aziridinate alkenes with iron catalysts in an asymmetric manner have met with only limited success to date [101], In an early report on the use of various chiral metal salen complexes, it was found that only the Mn complex catalyzed the reaction whereas all other metals investigated (Cr, Fe, Co, Ni etc.) gave only unwanted hydrolysis of the iminoiodinane to the corresponding sulfonamide and iodoben-zene [102], Later, Jacobsen and coworkers and Evans et al. achieved good results with chiral copper complexes [103]. 

A carbene or nitrene transfer reaction to a carbon-carbon or carbon-heteroatom double bond system leads to the formation of three-membered rings, such as a cyclopropane, an aziridine or an epoxide. These processes can be catalyzed by applying iron catalysts and the different cyclic systems are discussed here. 

The aziridination of olefins, which forms a three-membered nitrogen heterocycle, is one important nitrene transfer reaction. Aziridination shows an advantage over the more classic olefin hydroamination reaction in some syntheses because the three-membered ring that is formed can be further modified. More recently, intramolecular amidation and intermolecular amination of C-H bonds into new C-N bonds has been developed with various metal catalysts. When compared with conventional substitution or nucleophilic addition routes, the direct formation of C-N bonds from C-H bonds reduces the number of synthetic steps and improves overall efficiency.2 After early work on iron, manganese, and copper,6 Muller, Dauban, Dodd, Du Bois, and others developed different dirhodium carboxylate catalyst systems that catalyze C-N bond formation starting from nitrene precursors,7 while Che studied a ruthenium porphyrin catalyst system extensively.8 The rhodium and ruthenium systems are... 

In a reaction modeled on the use of cytodirtxne P-450 to catalyze oxidations with iodosylbenzene, iron or manganese porphyrins have been used to catalyze aziridinations with iodinanes (Scheme 22). In this early report cis- or rranr-stilbene each gave the rrans-aziridine, but stereoselectivity has since been achieved for the sulfonylaziridines (Section 3.5.2.6). 

Aziridination Catalyzed by Iron, Manganese and Copper Salts Simple and Substrate-Induced Diastereoselectivity... 

The same iron complex also catalyzes the reaction of imines and diazo compounds to give d -aziridines (Sch. 31) [142]. 

Alkynes and allenes also react with amides. Phenylthiomethyl alkynes were converted to A-Boc-A-phenylthio allenes with Boc azide and an iron catalyst. The palladium-catalyzed reaction of an allene amide, with iodobenzene, leads to A-sulfonyl aziridines having an allyhc group at Other allene A-tosylamines... 

Cycloaddition, Diels-Alder reactions, epoxidation of aromatic aldehydes, isomerization of aryl-substituted epoxides, and aziridination catalyzed by the iron Lewis acid [(j 5-C5H5)Fe(CO)2(THF)] 03ACA3. 

Imidoiodanes and especially A-tosyliminoiodanes, ArINTs (Section 2.1.12.4), have found broad synthetic application as useful nitrene precursors in transition metal catalyzed aziridination of aUcenes and amidation of various organic substrates [584, 761]. Mansuy and coworkers in 1984 first reported the aziridination of alkenes with tosyliminoiodane PhINTs in the presence of iron- or manganese-porphyrins [762]. This reaction has a mechanism similar to the metal-catalyzed oxygen atom transfer reactions of iodosylbenzene (Section 3.1.20) and involves a metal-nitrene complex as the intermediate. 

The aziridination of olefins has also been studied, but fewer complexes catalyze this reaction as efficiently as iron and manganese complexes catalyze the epoxida-tion of olefins. Nevertheless, the aziridinations of olefins catalyzed by copper, ruthenium, and rhodium complexes have been reported. The source of nitrogen is usually [N-(p-toluenesulfonyl)imino]phenyliodinane (PhI=NTs) or a precursor to a related iodoarylimine. The aziridine is likely generated from these copper- and rhodium-catalyzed reactions by an outer-sphere process in which the olefin interacts with the LUMO of the complex, which is located at the nitrogen. This mechanism is more likely to be followed by these catalysts than a [2-t-2] process, followed by reductive elimination. 

Aziridination of Alkenes. Iron- and manganese-porphyrin corrqilexes catalyze the reaction of PhI=NTs with alkenes to form the corresponding fV tosylaziridines. Mn(TPP)Cl is generally a better catalyst than the analogous iron complex, affording 80% of the aziridine from the reaction with styrene (eq 11). Good yields are also obtained in the manganese-catalyzed reactions with 1,1-and l,2-diphenylethylenes. Yields of aziridines derived from aliphatic alkenes remain low and are complicated by the formation of allylic amines. 

The use of manganese and iron porphyrins to catalyze NTs transfer from PhINTs to olefins to form aziridines (catalytic aziridination) was first described in 1984 (5)... 

Hossain et al have already found that the cyclopentadienyl dicarbonyl iron Lewis acid, [CpFe(CO)2(THF)] BF4 (108), catalyzes a variety of reactions including cy-clopropanation [34a], aziridination [34b], and Diels-Alder reactions [34c]. Recently, they have reported that the iron Lewis acid [CpFe(CO)2(THF)]+BF4 (108) catalyzed synthesis of 3-hydroxy-2- arylacrylic acid ethyl ester (106) [34d]. The product distribution of (108) is not affected in the presence of proton sponge, but is dependent on temperature and the nature of the substrate aldehyde. 

In 1998, Hossain et al. reported the catalytic synthesis of aziridines, using an achiral iron Lewis acid-THF adduct, [CpFe(CO)2(THF)]+[BF4] (108). The reaction was generally ds-aziridine selective. With this approach, cis selectivity is now known to be typical, since most catalytic reactions mainly yield ds-aziridine. In 2001, they continuously reported the reason for the apparent cis-aziridine selectivity in the reaction of ethyl diazoacetate (10) with JV-benzylidene aniline (109a), catalyzed by (108) [36]. The catalytic reaction produces both cis- and trans-aziridines. Once... 

Similarly, the iron complex catalyzed aziridination of aryl imines with diazoacetate [89] Phenyldiazomethane also formed the aziridination product in higher... 

Metal-catalysts that enable the direct transfer of an Ni-moiety from chloramine-T to alkenes with moderate efficiencies include metalloporphyrinoids. For example, in 2001, Simkhovich and Gross reported that iron (IV) corrole 21 (Scheme 2.29) catalyzed olefin aziridination with chloramine-T [44]. The use of porphyrine complexes of iron [Fe(TPP)Cl] and cobalt [Co(TDClPP)] in the presence of bromamine-T also allows for aziridinating a wide variety of alkenes [45,46]. A recyclable polymer-supported manganese (II) complex, which is readily prepared from chloromethy-lated poly(styrene-divinylbenzene) (PS-DVB) in two steps, transfers an Ni unit from bromamine-T to aliphatic alkenes in a heterogeneous system [47]. 

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